Question

Define molarity, molality and normality of a solution.

Answer 1

Hint: As we know that, molarity, molality and normality are all different units of concentration in Chemistry. We can define concentration as the amount of solute in a given amount of solution or solvent.

Complete answer: Now, let’s understand these three concepts as,
Molarity denoted by ‘M’ can be stated as the number of moles of a solute per liter of solution.
Molarity, M is equal to mass of solute per liter volume of a solution. The unit of Molarity is mol/L.
We can write the formula for the molarity as,
$Molarity\left( M \right) = \dfrac{{{\text{Moles of solute}}\left( {mole} \right)}}{{{\text{Volume of solvent}}\left( l \right)}}$
Whereas molality denoted by ‘m’ can be defined as the number of moles of a solute per kilogram of a solvent. Molality, m is the number of moles of a solute per kg weight of a solvent.
The unit of molality is mol/kg. We can write the formula for the molality as,
$Molality\left( m \right) = \dfrac{{{\text{Moles of solute}}}}{{{\text{Mass of solvent}}\left( {kg} \right)}}$
Then, we can define normality, N as the number of equivalents per liter of solution. Normality, N is the no. of equivalents of solute per liter volume of the solution. The unit of Normality is gram equivalent per liter. We can write the formula for the normality as,
$Normality\left( N \right) = \dfrac{{{\text{Gram equivalent of the solute}}}}{{{\text{Volume of solution}}}}$
Now, let’s compare the terms molality and molarity. It is convenient for us to use these terms in experiments with respective temperature change. This is applicable because the volume of a solution increases when the temperature increases, and with rise in temperature molarity tends to decrease. However, we can say that molality is based on the masses rather than the volumes. Thus, molality remains unchanged with respect to temperature.

Note: An important point that we need to keep in mind is that the concentration of a solution depends on two things i.e. solute and solvent. And the concentration of a solution is an important concept as it determines the rates of a reaction and the conditions occurring at equilibrium.